1. Field of the Invention
This invention relates to high T.sub.c superconductor films deposited on gallate substrates, and more particularly to such film-substrate combinations which are suitable for device structures at temperatures above 77.degree. K.
2. Description of the Prior Art
The discovery of high temperature superconductivity in copper oxide based materials by Mueller and Bednorz in 1986 has been followed by many advances based on their discovery. Thus, the superconducting transition temperature has been raised to approximately 125.degree. K. in a thallium-based copper oxide (S. S. A. Parkin et al., submitted to Phys. Rev. Lett., March 11, 1988). The initial discovery of a thallium-based superconductor by researchers at the University of Arkansas was announced at the World Congress on Superconductivity, held in Houston, Tex., Feb. 22-24, 1988. Just prior to this, researchers headed by H. Maeda at the National Research Institute for Metals, Tsukuba, Japan had announced a superconducting compound of bismuth, calcium, strontium, copper, and oxygen showing evidence of superconductivity at about 105.degree. K. A few days after this, Paul Chu at the University of Houston announced a substantially identical compound except that it contained an additional element--aluminum.
Prior to the discovery of the bismuth-based copper oxide and the thallium-based copper oxide superconductors, most of the work in this technology had been centered on rare earth copper oxides and in particular on yttrium barium copper oxide superconductors having a transition temperature of about 95.degree. K. These materials were typically known as 1-2-3 compounds because of the atomic ratio Y-Ba-Cu in the superconducting phase of these materials.
All of these copper oxide based superconductors contain sheets, or planes, of copper oxide which appear to be responsible for carrying the supercurrents. However, the bismuth and thallium-based compounds appear not to have the copper oxide chains which are present in the rare earth copper oxide superconductors and in the yttrium 1-2-3 compounds. One of the superconducting phases in the bismuth-based oxides appears to be 2-1-2-2 (2 bismuth atoms, 1 calcium atom, 2 strontium atoms and 2 copper atoms), while the thallium-based superconductor contains thallium, calcium, barium, copper and oxygen, the superconducting material possibly having a 2-1-2-2 as well as a 2-2-2-3 superconducting phase.
It is known in the art how to produce thin films of high T.sub.c superconducting material, and specifically epitaxial superconducting films. Such films have been made by several techniques including electron beam evaporation, sputtering, and solution pyrolysis. In particular, films of high temperature copper oxide superconductors have been produced on several substrates, including SrTiO.sub.3, Si, Y-stabilized ZrO.sub.2, MgO, Al.sub.2 O.sub.3, and various aluminates. To date, the best films have been deposited on SrTiO.sub.3, these films being produced epitaxially and with the highest critical current density. Articles generally describing thin film deposition of oxide superconductors include the following:
1. M. Nastasi et al., J. Mater. Res. 2 (6), p. 726, Nov/Dec. 1987. PA0 2. M. Naito et al., Ibid, p. 713 PA0 3. R. B. Laibowitz et al., Phys. Rev. B 35 8821 (1987) PA0 4. P. Chaudhari et al., Phys. Rev. Lett., 58 2684 (1987) PA0 5. A. Gupta et al., Appl. Phys. Lett., 52, 163 (1988).
Although many substrates have been tried for the preparation of high T.sub.c oxide superconducting films, the results to date have not been superior in all respects. For example, it has not been possible to deposit high quality epitaxial thin films which are superconductive in their as-deposited state. It has generally been the situation that the film was amorphous or fine grain polycrystalline as-deposited and crystallized in a high temperature post annealing step. This has been the method used to obtain the highest quality epitaxial, high current density films. Sometimes an annealing step is used to convert the film from tetragonal to orthorhombic. Since the lattice constants for these two crystalline structures are quite far apart, the required structural change has often produced superconducting films that contain crystalline twins, microcracks, and contamination. Thus, none of the presently known substrates has offered the possibility of direct deposition of a high quality, high current density, high T.sub.c superconducting film without the additional processing steps. This is especially true for the 1-2-3 films that are orthorhombic instead of having 4-fold planar symmetry such as the readily available substrates MgO, SrTiO.sub.3 etc.
As mentioned, SrTiO.sub.3 has so far produced the best superconducting copper oxide films. This substrate provides a reasonable lattice match to the superconductor films and can tolerate high temperature annealing steps. However, large quantities of Sr go into the superconducting film during high temperature processing steps, and for this reason the reactivity of this substrate is quite high. Further, and more importantly, SrTiO.sub.3 has an extremely high dielectric constant which is variable from sample to sample in accordance with the substrate orientation and temperature, this material also being a very lossy dielectric. It is difficult to produce using preferred conventional crystal growth techniques such as Czochralski or Bridgman techniques and costly to produce by the Verneuil method. In addition to the fabrication costs, it is very difficult to obtain large area substrates of SrTiO.sub.3.
Si is a useful material for semiconductors, but has a disadvantage in that it generally must be passivated by a thin layer prior to the formation of the high T.sub.c oxide superconductor. This is because the Si atoms tend to diffuse into the superconductor and adversely affect its high T.sub.c superconductivity.
MgO is a substrate material whose dielectric constant is very much less than that of SrTiO.sub.3 ; however, it is not favorable for high temperature annealing steps due to interdiffusion of Mg, and can be difficult and expensive to prepare for the deposition of a superconducting film thereon. More importantly, its lattice constants do not match well with superconducting copper oxide films so that epitaxy is not favorable without other techniques such as graphoepitaxy.
Y-stabilized zirconia is chemically better than MgO at high temperatures, but inferior from a dielectric loss viewpoint. Additionally, the lattice match to the copper oxide superconducting films is not favorable.
Aluminates have also been used as substrate materials, but these substrates are very reactive, and the reactivity increases as the processing temperatures increase. It has been discovered that Al will replace Cu detrimentally in the superconducting film to adversely affect the superconducting properties. Additionally, the spacing of atoms in the aluminates is too small to give good lattice matches to the superconducting film, regardless of the processing temperatures.
Thus, while thin films of high T.sub.c oxide superconductors have been made which in some instances have yielded very high critical currents, the presently known substrates all have disadvantages which may preclude the use of superconductive films in device structures. Accordingly, it is a primary object of the present invention to provide a class of materials which can be used as substrates and interface layers for the growth of films of high T.sub.c oxide superconductors having improved properties.
It is another object of the present invention to provide high T.sub.c copper oxide superconductor-substrate combinations wherein high quality, single crystal epitaxial superconductor films are produced.
It is another object of the present invention to provide high T.sub.c oxide superconductor film-substrate combinations suitable for use in electrical devices.
It is another object of this invention to provide high T.sub.c copper oxide superconductor film-substrate combinations which are particularly well suited for analog and digital signal processing devices.
It is a further object of this invention to provide a class of improved substrate materials for high T.sub.c oxide superconductors where the substrate material can advantageously be used as an interlayer insulator in multilayer high T.sub.c superconducting devices.
It is another object of this invention to provide epitaxial films of high T.sub.c copper oxide superconductors on substrates having good lattice matching and good electrical properties for device configurations.
It is another object of this invention to provide favorable substrates and interlayer insulators for perovskite high T.sub.c superconductors.
It is another object of this invention to provide improved perovskite superconductor-substrate combinations.
It is another object of this invention to provide superconducting perovskite-gallate combinations which can be used in superconducting electrical devices.
It is another object of this invention to provide highly oriented superconductive oxide films on gallate substrates.
It is another object of this invention to provide oxide superconductors-gallate substrate combinations which have substantially matched atomic spacings.